Hello Amanda,
I do not know if this information will be useful or not -
http://www.proni.gov.uk/ulstercovenantsearch.
The Ulster Covenant 1912 is showing a number of Diamonds. I've checked 11 men's names.
If you think of a few more check them at this site and post the ones with the Belfast addresses.
The results I obtained are as follows -
Michael Diamond - Portrush, Co. Antrim
Edward Diamond - Baden Powell Street, Belfast
Robert Diamond - 200, Antrim Road, Belfast
Samuel Diamond - 59, Crumlin Road, Belfast
The following either lived in the country or produced a nil result -
Albert, Alexander, James, John, Thomas and William
Best Wishes, Christopher